The boilers of coal-fired steam generators in electric utility plants are typically used in conjunction with crushers to reduce raw coal to chunks or particles of less than a certain size prior to pulverization and introduction to the boiler combustion chamber. To prolong the life of the crusher components, the coal is first passed through a sizing/segregating grid made up of parallel bars disposed in the vertical path of a coal chute and spaced apart to pass smaller chunks such that they may be diverted around, rather than through, the crusher. In a given coal stream, this segregation may result in between 40% and 80% of the coal bypassing the crusher.
The grid in the aforementioned apparatus is typically made up of steel bars welded into a grid and disposed in a vertical chute at an angle such that smaller chunks of material pass through while larger chunks of material are caught by the grid and directed into the crusher. In such an operation, some coal chunks are inevitably of such size as to collect on and between the grid bars. Ultimately, such collected materials jam or plug the grid in varying degrees and degrade the efficiency of the segregating operation. To ensure proper operation, these collected materials must be removed from the grid.
For this purpose it is known from my U.S. Pat. No. 4,966,689 to use a clearing comb made up of rigid steel fingers normally located in a home position below and behind the grid where it is essentially out of the path of travel of the material which passes through the grid. The comb is periodically actuated by an air cylinder or the like to cause the fingers to lift as a unit and to simultaneously pass upwardly through and between the grid bars to lift and clear the jammed materials from the grid. The air cylinder is then deactivated and the comb drops back to the home position.
While the comb fingers pass between the grid bars they effectively block the grid and divert all material into the crusher.
In my co-pending application Ser. No. 560,076, filed Jul. 30, 1990, I disclose a sizing apparatus for rough size-segregation of coal and other particulate material wherein the sizing/segregation grid is continuously cleared of material which jams between or bridges the bars of the grid. In general, this is achieved through the use of a rotating comb comprising a plurality of parallel fingers which rotate through and between the bars of the grid about an axis which passes essentially through and at right angles to the grid bars.
In an illustrative embodiment further disclosed therein, a sizing grid is equipped with two rotating combs, the fingers of respective combs rotating about spaced parallel axes and driven by a single motor. The fingers of each comb are progressively staggered in angular position and the two combs are arranged relative one another so that the swept volumes thereof overlap. Comb fingers are angularly arranged to avoid mechanical interference during rotation. In a preferred form, the comb fingers are staggered in angular relationship to one another or arranged in staggered groups so that the grid is never completely blocked over its entire area. In this fashion, the crusher is uniformly loaded at all times. In fact, the comb fingers may be arranged on two or more parallel, spaced axles which are synchronously driven. Moreover, means are provided for reversing the direction of rotation of the combs. In the illustrative embodiment, this is achieved on a demand basis by monitoring comb drive motor current and activating a reversing switch whenever an increase in motor current indicates a stalled or incipient stall condition in the motor.
Coal fed into the sizer-segregator and subsequently to the crusher typically contains a substantial amount of debris left over from the mining and initial processing. The debris can include relatively large pieces of wood, metal or rock, all of which can seriously damage the crusher and make inoperative various safety features in the crusher. It is known to provide a magnetic separator upstream of the sizer-segregator to remove ferromagnetic metal from the coal fed into the system. Magnetic separators, of course, fail to separate non-ferromagnetic junk such as wood, non-magnetic metal and rock, and therefore are an inadequate solution to the problem of keeping damaging debris out of the crusher.
The typical coal sizing/segregating grid as described above performs only a coal separating function; i.e., it permits small pieces of coal to bypass the crusher while directing larger chunks of coal directly into the crusher. Unfortunately, any large debris missed by the magnetic separator will also be directed straight into the crusher by the grid, creating the potential for expensive and time-wasting damage to the crusher.